I am, however, a bit confused how Incarnate solves CKurt's question. From what I could tell looking through the code, the service returns a absolute URI to the image of a user's avatar. If I comment on a site pulling in my avatar from Twitter, then go change
my avatar on Twitter, the comment I left won't have my updated avatar since it had an absolute URI to the old avatar. In fact, unless I'm mistaken, it might give a broken link, no?

The way to solve this would be to have Incarnate host the avatars used so no links ever get broken by 3rd-party services, but then Incarnate would have to do more heavy lifting.

Also, how is this using microformats? At least from Twitter and Facebook you seem to be parsing the profile pages to get a link to the avatar.